The present invention relates to method and apparatus for preventing off-track writing in a disk drive. More particularly the invention relates to inhibiting a write operation when the predicted position of a read/write head is off-track.
A magnetic disk drive is designed with a means of inhibiting write operation at an off-track position, i.e., the read/write head is not over the correct track, in order to insure that data is recorded on the correct track on the disk. To prevent an off-track write, there is a conventional method of utilizing a position error signal (PES) calculated from a signal obtained by reading servo burst patterns on the disk. In Published Unexamined Patent Application No. 6-243617 there is disclosed an example of a method of utilizing the PES.
Recently another method of preventing off-track writing has been described in which the sum (P+V) of the PES(P) and velocity (V) is used as a predicted PES and a comparison is made with this predicted PES as a reference value in order to allow or inhibit write operations. In this method, PES (Pn) and velocity (Vn=Pnxe2x88x92Pnxe2x88x921) are calculated from a value by reading the nth servo position. Based on the calculated Pn and Vn, the following predicted PESn+1 is calculated.
Predicted PESn+1=Pn+Vn
The read/write head is inhibited from writing data onto a track between the nth servo position and the (n+1) servo position, when the absolute values of the calculated Pn, Vn, and PESn+1 meet any condition of the following conditions (1) through (3):
|Pn| greater than C1xe2x80x83xe2x80x83(1)
|Vn| greater than C2xe2x80x83xe2x80x83(2)
|Predicted PESn+1| greater than C3xe2x80x83xe2x80x83(3)
where C1, C2, and C3 are constants.
However, in the above-mentioned method of utilizing a predicted PESn+1, if the predicted PESn+l significantly differs from the actual PESn+1, then the write operation may be unnecessarily inhibited, i.e., falsely triggered. For example, in the (n+1) servo position in FIG. 4(a), when the actual PESn+1 is present outside a range of the inhibition criteria with respect to a target track T and is present at a position where write inhibition is unnecessary, the predicted PESn+1 is within the range of inhibition criteria, as shown in FIG. 4(b). In such a case, unnecessary write inhibition is performed on the track between the nth servo position and the (n+1) servo position. It is believed that the reason for this is that while the output DAC of the servo control represents a position in a neutral state, the servo is always on and the output DAC of the servo control never goes to the neutral state. Conventionally, it is also known that the predicted PES is calculated with a state estimator. However, the state estimator requires a high-performance processor such as a DSP. If it is used in a storage unit such as a disk drive unit, the cost is too high. For this reason, this state estimator is not realistic in products having low cost as their object.
An object of the present invention is to provide a method for preventing off-track writing which reduces the false triggers (xe2x80x9coverkillxe2x80x9d) state caused by inaccurate prediction of the position of the head.
Another object of the present invention is to provide an improved method for preventing off-track writing without requiring a high-performance processor such as a DSP.
A disk drive with the capability to prevent off-track writing with reduced false triggering is described. An improved method of predicting the position of a head is used to determine whether the head is on or off-track. A position error signal at an nth servo position Pn and a velocity Vn at the nth servo position (Vn=Pnxe2x88x92Pnxe2x88x921) are calculated using values read at the nth and nxe2x88x921 head positions. Based on the calculated Pn and Vn and on the output of the servo controller at the nth servo position Un, the predicted PESn+1 is calculated as:
Predicted PESn+1=Pn+Vn+kUn
where k is a constant.
Data is inhibited from being written between the nth servo position and the (n+1) servo position, when absolute values of the calculated Pn, Vn, and predicted PESn+1 meet any condition of the following conditions (1) through (3): (1) |Pn| greater than C1, (2) |Vn| greater than C2, and (3) |Predicted PESn+1| greater than C3 where C1, C2, and C3 are constants determined for the embodiment.